Is human activity warming the Earth or do recent
signs of climate change signal natural variations? In this feature
article, scientists discuss the vexing ambiguities of our planet's
complex and unwieldy climate.

October
20, 2000 -- Newspaper headlines trumpet record-breaking temperatures,
dwindling sea ice, and retreating glaciers around the world.
Concentrations of atmospheric carbon dioxide, one of the greenhouse
gases responsible for scalding temperatures on Venus and at least
33 degrees C of normal warming here on Earth, are on the rise.
Our planet seems destined for a hot future!

But is it really? Or are we simply experiencing a natural
variation in Earth's climate cycles that will return to "normal"
in time?

Correlations between rising CO2 levels and global
surface temperatures suggest that our planet is on a one-way
warming trend triggered by human activity. Indeed, studies by
paleoclimatologists
reveal that natural variability caused by changes in the Sun
and volcanic eruptions can largely explain deviations in global
temperature from 1000 AD until 1850 AD, near the beginning of
the Industrial Era. After that, the best models require a human-induced
greenhouse
effect.

In spite of what may seem persuasive evidence, many scientists
are nonetheless skeptical.

They argue that natural variations in climate are considerable
and not well understood. The Earth has gone through warming periods
before without human influence, they note. And not all of the
evidence supports global warming. Air
temperatures in the lower atmosphere have not increased appreciably,
according to satellite data, and the sea ice around Antarctica
has actually been growing for the last 20 years.

It may surprise many people that science -- the de facto
source of dependable knowledge about the natural world --
cannot deliver an unqualified, unanimous answer about something
as important as climate change.

Why is the question so thorny? The reason, say experts, is
that Earth's climate is complex and chaotic. It's so unwieldy
that researchers simply can't conduct experiments to check their
ideas in the usual way of science. They often rely, instead,
on computer models. But such models are only as good as their
inputs and programming, and today's computer models are known
to be imperfect.

Most scientists agree that no single piece of data will likely
resolve the global warming debate. In the end, the best we can
expect is a scientific consensus based on a preponderance of
evidence.

About
85 percent of Greenland is covered by a massive ice sheet with
an area of about 1,736,000 square kilometers and an average thickness
of about 1,500 meters. The volume of ice in the Greenland
sheet is estimated to be about 2,600,000 cubic kilometers --
enough ice to raise sea levels by 6.4 meters if it all were to
melt.

While it is only about one-seventh the size of the Antarctic
ice sheet, some scientists think that watching the ice on Greenland
provides better clues about global warming.

"Even though Antarctica is seven times the size of Greenland,
because (Antarctica is) kind of symmetrically positioned around
the South Pole, it doesn't really interact with climate up in
the more temperate regions the way Greenland does," said
Dr. William Krabill at NASA's Wallops Flight Facility. Krabill
is the project scientist for the team that discovered the thinning. "Greenland
... is likely to be a better indicator of global climate change
than Antarctica," he noted.

Right: A map of Greenland showing
the changes in surface elevation measured by Krabill's team.
Gray indicates no change, white indicates an increase in thickness
and shades of blue indicate a decrease in thickness, with darker
blue denoting greater thinning. [more
images]

Krabill's team used an airborne laser to survey the altitude
of the ice sheet's surface during 1993 and 1994. They repeated
their survey in 1998 and 1999, making certain to retrace their
flight paths from the first survey as closely as possible.

After incorporating some assumptions that let them extend
their measurements to the sheet's edges, the scientists compared
the second survey to the first. They found that the ice sheet's
surface was slightly higher at the center but considerably lower
at the edges -- particularly the southeastern edge.

The overall result: The ice sheet lost at least 51 cubic kilometers
of volume during that five year period. Greenland appeared to
be melting!

Many newspaper headlines cried the discovery as a sign of
global warming -- which most readers presumably took to mean
"anthropogenic," or human-caused, global warming.

But is that the right conclusion?

"What you can say is, yes, carbon dioxide (in the atmosphere)
is at levels higher than ever before, and carbon dioxide is a
greenhouse gas, so it's reasonable to say that there's warming
associated with the increase of carbon dioxide," said Dr.
Waleed Abdalati, co-author of the paper that announced the Greenland
discovery.

"But you can't make the leap yet that all the cars in
the world have led to what we're observing in the thinning of
the Greenland ice sheet," Abdalati said.

Left: A graph showing the Earth's average
global surface temperature from 1880 to the late 1990s. The black
dotted line follows the averages for each year and the red line
traces the five-year average. There is a clear upward trend over
that time.

If there's one lesson to be learned from science, it's that
things are usually much more complex than they at first appear.
The warming trend of the last century may seem to be the obvious
explanation for the thinning seen on Greenland, but scientists
are considering other possibilities.

"That's what science is about," said Dr. Ellen Mosley-Thompson,
a research scientist at the Byrd Polar Research Center at The
Ohio State University.

"Just because you have an hypothesis and immediately
your experiment produces support for it, you can't simply accept
those results (without a degree of skepticism)," Mosley-Thompson
said. "The whole idea is to play devil's advocate on your
own research before your colleagues do."

Last century's warming trend is not the only possible explanation
for the thinning that Krabill's team saw on Greenland.

In fact, ice cores taken as part of another NASA-funded study
suggest that natural variation in snowfall may be partly to blame,
Mosley-Thompson said.

"The ice core data provide evidence -- not necessarily
conclusive -- that Bill's results may in part reflect variability
in snow accumulation over his five-year observational window,"
said Mosley-Thompson, who co-authored the paper reporting these
results with Dr.
Joseph McConnell, an associate research professor at The
Desert Research Institute in
Reno, Nevada. The results of the study were published in the
August 24 edition of the journal Nature.

Other natural processes could account for the thinning as
well. Ocean currents might have caused part of the change. Or
the flux of warm water into the North Atlantic caused by the
1990-1996 positive phase of the slow-moving North
Atlantic Oscillation could have had an influence. The ice
sheet could also be thinning in response to the long-term warming
of the planet since the transition from the last glacial period
about 10,000 years ago. Krabill, Dr. Ron Kwok of NASA's Jet Propulsion
Laboratory, and Abdalati mentioned these scenerios during interviews
with Science@NASA.

Scientists often refer to these alternate explanations under
the umbrella term of "natural variability."

The ant on the hour hand

"For the ordinary person, it's a common misperception
that weather is not changing ... that last winter is about as
cold as this winter and last summer is about as warm ... and
the world is pretty much constant," Krabill said. "That's
not true. The Earth has gone through and continues to go through
cycles of warming and cooling. It's just natural."

This natural variability often shows an astounding degree
of complexity, much of which remains poorly understood.

"We've only begun making (large scale) measurements in
the last 100 to 150 years," Abdalati said. "And climatic
processes happen on very different time scales. There are some,
like ice ages, that are in the tens of thousands or hundreds
of thousands of years long. An then there are atmospheric processes
like weather, which happen on the scales of hours and days."

Other climate cycles fall in between,
such as the North Atlantic Oscillation mentioned above, which
is thought to complete one cycle roughly every 20 to 30 years.

"And so you have all these processes mixed together that
have been going on for thousands of years, and you're in the
difficult position of trying to separate something very recent
from the natural cycle without fully understanding what that
natural cycle is," Abdalati said.

Left: Knowing where a relatively
short interval of observation fits into the long-term pattern
is a difficult challenge for scientists. A steady increase that
appears to be a trend may be a trend, but it may also be a small
part of a larger cycle.

Observing a system like climate that varies on several time
scales -- some of which approach geological slowness -- could
be likened to an ant watching the hands of a clock, "perhaps
with the ant sitting on the hour hand," Abdalati added.

Seen in this context, scientists don't give much weight to
the five-year snapshot of the ice on Greenland.

"You know, five years is a pretty short amount of time
in glaciological terms," Krabill said. "To try to make
inferences about 'Global Climate Change' in capital letters from
a five-year period of time is a pretty risky business."

Other modern data sets are not much longer. The era of satellite
observation is only about 30 to 40 years old -- a mere blink
in climatological terms. And the widespread network of weather-measurement
stations in the developed world is about 150 years old.

The Ghost of Climates Past

Greater insight about the role of natural variability may
come from the field of paleoclimatology -- a specialized branch
of climatology that uses scientific sleuthing to summon the ghost
of climates past.

The
"fingerprints" of Earth's climate hundreds or even
thousands of years ago remain imprinted in the rings of temperature-sensitive
trees, the chemicals trapped in ancient ice, and the layers of
sediment on the ocean floor.

Several studies by paleoclimatologists have suggested that
natural variability can't fully explain the warming of the last
century.

For example, Dr.
Thomas J. Crowley, a geologist at Texas A&M University,
used similar techniques to reconstruct basic climate data --
such as average global temperature -- back to 1000 A.D.

Crowley examined natural climate variations in a simple computer
climate model caused by two external influences: fluctuations
in the sun's intensity and aerosols injected into the atmosphere
by volcanoes.

He deduced the history of solar flux from concentrations of
carbon-14 in tree rings and of beryllium-10 in ice cores. Then,
he deciphered past volcanic activity from sulfate aerosol deposits
in ice cores.

Crowley ran the computer climate model with the solar and
volcanic forcing terms, then compared the average temperatures
it produced with a temperature record constructed from tree-ring
data.

Despite the relative simplicity of his model, Crowley found
good agreement between the temperature fluctuations it calculated
for the years 1000 AD to 1850 AD and the fluctuations actually
measured from tree rings during that interval. Over that 850-year
period, fluctuations in solar intensity along with volcanic eruptions
could account for roughly 50 percent of the variation seen in
the tree-ring record -- give or take 10 percent.

Something happened, however, after 1850. Crowley's model could
only account for about 25 percent of the observed temperature
changes. Something else was needed -- volcanic eruptions and
solar variability were not enough.

Crowley then introduced a human-triggered greenhouse effect
to the model and it produced a much better match.

"It all comes out as indicating that you can't resort
to (natural variability) to explain the recent warming,"
Crowley said. "The (recent) warming is consistent with a
greenhouse effect but inconsistent with any explanation from
natural variability."

So with the weight of a 1,000-year climate record on human
shoulders, can scientists finally say that they've proven
humanity is causing an unnatural warming of the globe?

Not necessarily.

"The time series we developed is statistically significant
-- highly significant," Crowley said. "That doesn't
prove something is right, but it still makes a good case
that we're on the right track."

The problem with the "P" word
--Proof!

From a statistical point of view, no single scientific result
based on real-life data ever deserves absolute confidence. There
always lingers the possibility -- however small -- that the apparent
results are due to chance patterns in the data, i.e.,
"noise."

In the case of Crowley's study, statistical tests show that
the probability of his results being due to chance is less than
1 percent. Usually, anything less than 5 percent is considered
credible.

"This is not mathematics where you can prove something
and write Q.E.D. at the bottom of the page," Crowley said.
"This is geoscience. It's a dirtier field, and usually you
make statistical arguments."

The abbreviation stands for the Latin phrase quod erat
demonstrandum, which means, "which was to be demonstrated."
Mathematicians use "Q.E.D." to indicate the end of
the written proof of a theorem, which, if correct, is considered
absolute.

Above: The general trend of
average global temperatures from Precambrian time -- when multicellular
organisms first evolved -- to the present. Notice that average
global temperatures vary by roughly 10 degrees Celsius between
warm and cool periods. Image courtesy of Dr.
Chris Scotese.

In addition to the caveats inherent to statistics, conclusions
from studies like Crowley's that are based on computer simulations
of the world's climate are plagued by questions of how well computer
models portray the real thing.

To prove causation, scientists must perform experiments under
controlled conditions on the system being studied, manipulating
the system to understand what causes what. Other
scientists repeat the experiments to show that the explanation
is reliable.

Since the Earth's climate is beyond the reach of such experimentation,
scientists instead run computer simulations of global climate.
These models are always much simpler than the Earth's climate
itself. In fact, it's theoretically impossible to create a "perfect"
model of climate that includes all the detail of the real system.

"The climate system is too complex," Mosley-Thompson
said. "Even the most complex climate model doesn't get it
right. And why is that? Because who writes the climate models?
Humans. What is a climate model? It's a set of equations that
describes what we think we know. If you're not cognizant of a
particular phenomenon, then how can you incorporate it into a
climate model?"

The fact that different computer models often produce different
forecasts doesn't offer much reassurance. For example, one model
predicted that the Southeastern U.S. would become more jungle-like
in the next century, while another model predicted the same region
would become a dried-out savanna, according to Dr.
John Christy, a professor of atmospheric science at the University
of Alabama in Huntsville.

However, scientists can establish some degree of confidence
in their computer models by seeing if the model can accurately
"predict" past climate patterns that are known to science.

"Models in isolation may not be believable, but when
... a model can simulate a number of different observed climate
responses, the results have more weight than mere calculation,"
Crowley said. "That still doesn't prove the point, but it
minimizes the value of the argument, 'It's only a model.'"

Putting the pieces together

Ultimately, the verdict from science about the extent and
cause of global climate change may not come from one particular
study or observation.

"I think from the viewpoint of thoughtful scientists,
there's not going to be any single indicator, but rather there's
going to be a concurrency of lots of indicators that's going
to be convincing," said Dr.
Tony Rosenbaum, a professor at the University of Florida
who specializes in the politics of environmental issues.

Above: Much of the energy coming
from the Sun is in the form of short-wavelength radiation, which
passes through the atmosphere. The Earth's surface re-radiates
that energy as long-wavelength radiation -- such as infrared
-- which is trapped by greenhouse gases, primarily water vapor,
carbon dioxide and methane.

While the "big picture" view of all the evidence
from research around the world may offer scientists their best
chance to understand global warming, no mathematical tools exist
for combining all the data into a definitive, objective conclusion. Scientists'
only option is to weigh the evidence and make a professional
judgment.

"There is a dichotomy between what is realistic and achievable,
and what some people would like to hold as the ideal proof,"
Mosley-Thompson said.

"I don't think this discussion lends itself to standard
statistical testing," she continued. "Certainly you
can test, statistically, the output of one climate model against
another, a climate model against observation, an ice core data
against observation -- but those are snippets .... What does
it mean for reality? There's where the translation becomes difficult."

When drawing a conclusion from the mosaic of evidence, different
scientists will use different criteria, Crowley said.

"I think that there are many scientists that are still
locked into the idea that we have to prove something (about causes
of climate change) beyond a reasonable doubt," Crowley said.
"I don't think we necessarily have to do that. ... In a
court of law you can convict based on a weight of evidence. ...
That's the way you approach this type of problem."

Because such conclusions are based on a scientist's professional
judgment, disagreement is inevitable.

"There is enormous room for differences of opinion among
equally competent scientists of good will," Rosenbaum concluded.

"There are always people -- and reasonable people --
who fall on both sides of the argument," agreed Abdalati.
"And there are reasons for that. So the best we can hope
for is a consensus."